The long-term objective of the proposed research is to develop hypotheses about the role of nutrition during lactation that are relevant to and ethically testable in human populations. The experiments planned will explore in an animal model, the laboratory rat, the effects of protein-energy malnutrition on the physiology of lactation. In a series of factorial experiments, the following hypotheses will be tested: (1) malnutrition has a significant effect on levels of hormones known to be important for galactopoiesis in the rat; (2) malnutrition has a significant negative effect on the normal lactational hypertrophy of the mammary glands, liver, and gastrointestinal tracts; (3) malnutrition has a significant negative effect on cardiac output and blood flow to the mammary glands; (4) beyond any effect on blood flow, there is little additional effect of malnutrition on nutrient uptake by the mammary glands; (5) previously malnourished animals refed during lactation do not differ significantly from controls, and (6) chronically malnourished rats are significantly differently affected by their dietary treatment than those first underfed during lactation. The fifth hypothesis is an important one to evaluate because of its relevance to possible public health interventions. The sixth hypothesis investigates an unexpected and physiologically interesting observation from our previous work. In a dose-response experiment, the following additional hypothesis also will be tested: in chronically malnourished rats, the relationship between the proportion of ad libitum intake fed and the hormonal or cardiovascular response to lactation is non-linear. This last hypothesis investigates our observation that there is substantial buffering of the deleterious effects of chronic malnutrition in animals fed amounts of food similar to those obtained by malnourished women in developing countries and, thus, is of both public health and physiological interest. Techniques to be used in the proposed research include our model of acute or chronic malnutrition in the lactating rat as well as relevant methods developed by others for the study of cardiac output, organ blood flow, nutrient uptake, and hormonal milieu in the rat.